The present invention relates to a combined cycle power plant. More specifically, the present invention relates to method and apparatus for warming a steam turbine in a combined cycle power plant prior to generating power in the steam turbine.
The low capital cost, short lead times and flexibility of gas turbine-based power plants make them particularly attractive to electrical utilities as a means for generating electrical power. Unfortunately, the inefficiency of a gas turbine standing alone, referred to as a simple cycle system, is relatively low compared to conventional fired boiler steam turbine systems.
Consequently, substantial effort has been expended in developing methods for recovering the energy available in the gas exhausting from a gas turbine. One of the most successful methods involves the transfer of heat from the hot exhaust gas to pressurized feed water in a heat recovery steam generator ("HRSG"). The HRSG generates steam that is expanded in a steam turbine, thereby producing additional rotating shaft power. Power plants employing such a heat recovery method are termed combined cycle power plants.
During start-up of the gas turbine, there is a relatively rapid increase in the flow rate of the hot gas exhausting from the gas turbine as it accelerates to operating speed. Thereafter, the exhaust gas flow rate remains relatively constant, except for the effect of compressor vane modulation. However, after the gas turbine reaches operating speed, the temperature of the exhaust gas gradually increases as the firing temperature of the gas turbine is increased up to the level required to produce the desired power output, which is typically the maximum continuous rated power output of the gas turbine. Generally, the firing temperature is increased as rapidly as possible given the constraints imposed on the rate of the temperature increase by the components in the gas turbine and the HRSG exposed to the flow of hot gas.
Although the hot exhaust gas from the gas turbine typically flows through the HRSG during the gas turbine start-up, a considerable period of time elapses before an initially cold HRSG is capable of generating steam at sufficient pressure and temperature--typically, at least approximately 1400 kPa (200 psi) and 370.degree. C. (700.degree. F.)--to initiate roll off and warming of the steam turbine rotor. Introducing low pressure, low temperature steam into the steam turbine could result in undesirable condensation within the steam turbine.
Traditionally, therefore, steam produced by the HRSG during start-up of the gas turbine is dumped to the condenser until such time as the HRSG is capable of generating steam at the appropriate pressure and temperature for introduction into the steam turbine. Unfortunately, this approach increases the time required to bring the steam turbine on line, since it delays warming of the steam turbine until the gas turbine has been operating for some time.
This situation is further complicated in a combined cycle power plant in which the gas turbine and steam turbine rotors are coupled to a common electrical generator. In such situations, the steam turbine rotor typically accelerates along with the gas turbine rotor. However, if no fluid is flowing through the steam turbine flow path because the HRSG is not yet producing steam at the appropriate conditions for introduction into the steam turbine, the rotation of the steam turbine rotor will result in overheating of the steam turbine blades due to heat up of the air trapped within the steam turbine flow path. Consequently, in such arrangements, the steam turbine flow path must be ventilated until the HRSG is generating steam that can be introduced into the steam turbine, thereby further complicating the start-up of the power plant.
It is, therefore, desirable to provide a simple and effective approach for rapidly warming a steam turbine during the start-up of the gas turbine and HRSG in a combined cycle power plant.